Intraocular pressure (IOP) is measured as an indicator of glaucoma and other conditions of the eye. IOP indicates the pressure that is exerted by the ocular fluid called “aqueous humor” that fills the anterior chamber of the eye. Normal IOP is in the range of 10-21 mmHg. Elevated IOP is associated with loss of optic nerve tissue, loss of peripheral vision, and leads to blindness if not treated. IOP measurement, optic disc examination, and visual field testing are used for glaucoma diagnosis. Regular monitoring of the above three parameters is highly useful for disease detection and management. Early treatment helps to slow disease progression. However, early signs are detectable only by a physician.
Conventional quantification of IOP measures the resistance of the cornea to indentation or applanation, using a device such as the Goldmann Applanation Tonometer (GAT). The GAT measurements are accurate to within 0.5 mmHg for IOPs of 20 mmHg or lower. While this provides some measure of accuracy in measuring IOP for most individuals, however, the GAT device has a number of shortcomings that limit its effectiveness for diagnosis and monitoring purposes. The GAT can only be used in the doctor's office, making it unsuitable for overnight use or longer term monitoring. Central corneal thickness affects the measurement of IOP by the GAT device. A thinner cornea than normal would applanate to a higher degree than normal, thereby providing underestimation of the pressure. Similarly, a thicker cornea than normal would provide an overestimate of the IOP.
Significantly, the GAT device and other instruments that measure IOP are not suitable for continuous IOP measurement. IOP fluctuations are known to occur but are not readily detectable using standard office measurement practice. Thus, there is a need for a measurement apparatus that allows monitoring of patient IOP over a period ranging from a few hours to a day or longer.
There have been a number of attempts to address the need for more portable IOP measurement devices, including those using various types of contact lens or other wearable device for noninvasive measurement. One type of device forms a wire strain gauge on an intermediate medium, affixes the intermediate medium within a semirigid holder or contact lens, and obtains periodic measurements therefrom. The strain gauge may use a Wheatstone bridge, LC circuit, or other arrangement of devices to provide a signal that indicates changes in the intraocular pressure measurement of the wearer.
Although various types of wearable device have been proposed, however, significant difficulties remain. Even where the proposed solutions are workable, for example, high complexity in manufacturing leads to high cost. Significantly, the proposed solutions do not readily lend themselves to customization, so that lenses made using these methods are difficult to fabricate for individual patients. Fabrication of a custom contact lens or other wearable device for IOP measurement can thus take considerable time, causing delay in obtaining measurement data and further driving up the cost of the measurement device. Furthermore, the accuracy of existing strain-gauge solutions and their suitability for full-fledged diagnostic use has also been questioned.
Thus, it can be seen that there is a need for apparatus and methods for fabricating a wearable lens that can be inexpensively manufactured and customized to provide accurate, noninvasive 24 hour monitoring of IOP for a patient.